Mr. Trump considers redirecting funds committed to rebuilding Puerto Rico toward the building of the Wall. From WaPo:
Trump has urged the Army Corps to determine how fast contracts could be signed and whether construction could begin within 45 days…
The list includes dozens of flood control projects in areas affected by recent natural disasters, including the Texas coastline inundated by Hurricane Harvey and parts of Puerto Rico battered by Hurricane Maria. The military construction budget is also being looked at as a potential source for unspent funds, with billions more potentially available there.
It might be that Mr. Trump believes that only 60 odd people died as a result of Hurricane Maria, instead of the official estimate of nearly 3000, so perhaps he can be excused for sheer willful ignorance.
More likely, he just likes the idea of taking money from suffering brown people to build a wall to punish (even if ineffectively) brown people. For him, it’s win-win.